This is indeed the speed rigorously derived by Fraenkel [1]}.
Vortex rings have been analyzed in larger generality in [2]}, [3]}, [4]}, [5]}.
Expression
(REF ) corresponds to a special case of formal asymptotics
obtained by Da Rios in 1906 [6]}, [7]},
of thin vortex tubes around curves evolving by their binormal flow.
See [8]}, [9]}, [10]}. Helicoidal travelling vortex tubes have been recently found in [11]}.
